1. Field of the Invention
The present invention relates to a Ca-containing rust-resistant steel, containing Ca at a concentration of about 5 ppm or more. In particular, it relates to the concept of effectively reducing the formation of rust on surfaces of steel products, such rust being caused by CaO-containing oxide inclusions in the steel products, thereby solving a problem which is often of concern when Ca or Ca alloy is added to molten steel.
In accordance with this invention, the formation of large cluster-like inclusions, which would otherwise result due to the low melting point of the oxide inclusions, is prevented.
This invention further prevents nozzle clogging during the associated continuous casting process, and reduces deformability during hot rolling of sulfide inclusions.
2. Description of the Related Art
There has long been a demand for common steels such as low carbon steels and extremely low carbon steels, as well as for various stainless steels, particularly thin steel sheets, to achieve an esthetically pleasing exterior surface appearances. These steel products can usually be processed into low carbon steels by deoxidation treatments using AI, Ti, or Si to combine with oxygen in the steel. However, it is still impossible to avoid the formation of deleterious amounts of oxide inclusions in the steel products treated by Al-deoxidation or Ti-deoxidation. In fact, the oxide inclusions usually contain as their main components Al2O3 and Ti oxide, each in an amount corresponding to an oxygen content of 10 to 80 ppm.
The above oxide inclusions, mainly containing Al2O3 and Ti oxide, are likely to adhere to and thus form deposits on the internal surfaces of an immersion nozzle of the type usually employed in a continuous casting process for injecting molten steel from a tundish into a mold. As a result, the nozzle is likely to clog, making it impossible to ensure the performance of a stable casting process.
Moreover, flakes of deposited materials tend to come off the nozzle and mix into the steel product, resulting in defects in the physical properties of the steel product. Furthermore, due to the clogging of the continuous casting nozzle, the flow of the molten steel becomes deflected within the casting mold, thus resulting in the problem in that particles on the surface of the molten metal within the casting mold will be undesirably mixed into the molten metal. In addition, since Al2O3 and Ti oxide tend to form cluster-like inclusions, which usually remain on the surface of a thin steel sheet, striped defects form on the surface of the steel sheet, making it difficult to achieve a good external appearance.
In order to overcome these problems caused by Al2O3 and Ti oxide, it has been suggested to add Ca to molten steel treated by Al-deoxidation, to form an oxide composition consisting of CaO and Al2O3. Such methods are disclosed in, for example, Japanese Unexamined Patent Application Publication Nos. 61-276756, 58-154447, 6-49523).
However, it has been found that if Ca is added and is caused to react with Al2O3, some oxides having low melting points also form. These low melting oxides contain CaO.Al2O3, 12CaO.7Al2O3 and 3CaO.Al2O3 as their main components.
However, problems arise in MnS steels, including HIC resistant steels, and in steels required to possess a desired burring characteristic, and other steels containing MnS. Since the MnS contained in the steel tends to assert harmful influences on the desired HIC resistance and burring properties of the steel, it has also been suggested that Ca be added in order to inhibit the formation of MnS (For example, Japanese Unexamined Patent Application Publication No. 56-9317).
However, whenever Ca is added to a molten steel, the added Ca also reacts with the S contained in the steel, undesirably forming CaS, which causes the subsequent formation of rust on the surfaces of the steel products.
In an attempt to solve these complicated problems, Japanese Unexamined Patent Application Publication No. 6-559 has suggested that the Ca content of a steel product be controlled within a range of 5 ppm to less than 10 ppm, so as to prevent the formation of rust on the surface of the steel product. However, even though the Ca content is controlled to a value which is less than 10 ppm, if the composition of the oxides remaining in the steel is not specifically controlled, and if the CaO concentration in a resulting inclusion is high, the problem remains that CaS cannot be prevented from forming in the areas around the CaO-containing oxides that remain in the steel, hence forming numerous starting points at which rust will subsequently develop.
As a result, the time period before rust forms on the surface of a steel product (after the steel product has been produced) is undesirably shortened, thus increasing the amount and speed of rust formation, rendering it impossible to avoid deterioration of the external appearance of the steel products.